The invention relates to a method for controlling an electrical light source by pulse width modulation. Exterior lighting in motor vehicles is realized according to prior art by means of filament lamps, discharge lamps or light emitting diodes (LED). With the pulse width modulation via a switching means, which as a rule is a power-MOSFET, a supply voltage is transferred to the light source, whereby the supply voltage or a parameter dependent thereon, such as for example the current or the electrical power, is measured and the pulse width is controlled as a factor thereof.
Contrary to the very slow filament lamps or gas discharge lamps LEDs have a very fast response time, what on the one hand favorably affects the signal effect, for example as a brake light, on the other hand in case of fluctuations of the mains voltage leads correspondingly fast to visible modulations in the luminous intensity. Just for motor vehicle applications the luminous intensity must be adhered to as constantly as possible, however, if due to the low supply voltage and the high load fluctuations in vehicle electrical systems partially not negligible voltage fluctuations appear at the illuminants.
A regulation of the actual luminous power with regeneration is practically not possible, as the permanent detection of the luminous power for vehicle applications is inappropriate from the technical as well as from the financial point of view. A regulation of the supplied electrical power at the illuminant, in fact, can be realized more easily from the technical aspect, however, it nevertheless partially leads to not insubstantial differences of the actual luminous power. With a mere supply voltage related control without regeneration the differences are still stronger.
From US-A1-2002 047624 an illumination unit for a portable phone is known. The brightness and the coloring of the display of the portable phone is stabilized, whereby voltage supply is effected via a battery arranged in the phone.
From US-A1-2004 0095185 a control of several LEDs is known. It is disclosed to keep the current, which feeds the LEDs, to keep constant. For this purpose a current stabilization is effected.